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  1. Dear Netweather Readers, Apologies for my absence from this site during the last few months, but now I am back! I have had to concentrate on passing my AAT Advanced Diploma in Accounting, there was some quite tough stuff which I needed to learn so I could pass final exams. Thankfully I now have the certificate! I am endeavouring now to make a prediction for the UK Autumn 2017, which is a little belated now it is the 9th September. In the autumn months, the upper Westerlies strengthen and the main baroclinic zones become more defined as the Arctic cools- this makes longer-range forecasting a little easier. Currently, these are the main macro-scale weather-patterns that we are looking at: 1) The eastern (and mid) equatorial Pacific is cooler than usual by up to 2˚C, judging by from the latest UNISYS temperature-anomaly charts. This is indicative of a weak Lá Niná type situation. Cool conditions over such an extensive area of equatorial waters would present as a weakening influence on the Intertropical Convergence Zone (ITCZ), this is likely to present as such when the ITCZ moves over the equator by about November and it is interesting to note the relatively cool waters over the Gulf of Guinea. However, easterly trades blowing from the cool eastern Pacific onto the relatively toasty western Pacific would be expected to be stronger than usual- with extra westerly momentum imparted to the atmospheric circulation leading to a stronger Hadley Circulation and stronger westerlies in higher latitudes. 2) The Atlantic Hurricane season is currently very active, with some powerful storms such as Hurricane Irma that devastated the Caribbean. Very strong easterlies on the northern side of hurricanes (and typhoons in the Pacific- these have also been active) feed westerly AAM into the Northern Hemisphere atmospheric circulation (unless these re-curve well north of the ITCZ so that the very strong westerlies south of the centres of such hurricanes remove westerly AAM from the Northern Hemisphere Circulation). Hurricanes also readily add warm and moisture- and energy- to depressions crossing the North Atlantic where these push north into mid-latitudes. Increased hurricane activity in the North Atlantic therefore tips the balance towards warmer and wetter conditions in Britain (particularly in September/October)- but there might not be much in the way of “wetter” for other macro-scale considerations outlined below. 3) The Quasi Biennial Oscillation (QBO) is back into “easterly phase” after close-on two years of continuous westerlies high over the equator. Average wind-speeds at the 30 mb level in August were 14.4m/s (over 30 mph) from the east; this is likely to impact the Northern Hemisphere in the coming few months- so the impact on our weather will show up in late October-November. An easterly QBO has, in past years, been linked with slightly weaker higher-latitude westerlies overall and an increased likelihood of cold airstreams reaching Britain from the Arctic or northern Russia in the winter half-year. It must be pointed out, however, that air at the 30 mb level is less than one thirtieth of the density it is at sea-level; thus whilst the QBO has a small impact on the atmospheric general circulation via momentum exchanges between it and the surrounding air- the impact will not be great. 4) The Sun is entering a really quiet phase as we draw towards the end of the current Sunspot Cycle. Current Solar Activity forecasts suggest few (if any) sunspots or Solar Flares over the coming month. The Solar Wind has dropped recently from 500 m/s to just above 300 m/s. There is a link between the intensity of upper lows in the sub-arctic and solar activity in the autumn and winter months. When the Sun is quiet this means weaker depressions. The Sun has been quiet for a few months now and all indications are that this will remain the case for the foreseeable future. 5) Sea surface temperatures at the beginning of September have been 1 to 2C above normal in the North Atlantic compared to the long-term norm for this time of year; the North Atlantic “Cold Patch” appears to have warmed out. Meanwhile, Arctic sea-ice extent is close to the long-term normal in the Atlantic/Greenland sector of the Arctic even though it is well polewards of it’s normal extent on the Alaska side of the Arctic. This pattern would dictate for stronger baroclinicity and more intense storms crossing the North Atlantic as we head towards winter. 6) Upper air forecasts for the next three weeks point to the upper-westerlies over the sub-arctic remaining considerably weaker than normal (see weatheriscool website). This strongly suggests a quieter-than-normal pattern with more high-pressure is likely for the remainder of September. In terms of importance to our weather in Britain factors (5) and (2) have several times the impact of factors (3) and (4); the likely impact of the ENSO Phase is more than that of a quiet Sun or the QBO turning easterly. For the next month factor (6) is liable to provide the strongest indication of how the weather is liable to play out, though the impact of more North Atlantic hurricanes is not one that should be dismissed. With all this in mind, the remainder of September is likely to be a little warmer than usual with plenty of fine sunny conditions over central and southern England, Yorkshire and South Wales. Daytime temperatures above 21˚C can be expected from time to time as high-pressure builds north and east over England. However, at this time of year, clear night skies inevitably mean falling temperatures and minima of 5˚C or below can be expected to occur widely across England on at least a few nights during September with early-morning fog and ground-frost likely in valley-bottoms. For Scotland, North Wales and north-west England along with Northumberland and Northern Ireland the baroclinic set-up and Atlantic hurricane activity is likely to result in a more unsettled flavour to the remainder of this month. Depressions passing to the north will bring spells of rain, strong winds from the west will be frequent and daytime temperatures will often remain the cool side of 15˚C, even in the lowlands. However, even in Scotland and the North there will be fine spells with ridges of high-pressure between depressions with maxima edging towards 20˚C but with the clear nights becoming cold enough for ground-frost at times. Frontal influences will also extend further south to central/southern England and South Wales to bring some days of rain, stronger winds and maxima below 18˚C. I do however, expect there to be more sunshine and warmth and less rain than usual for central and southern England on the whole. For Scotland and the north-west of England, along with Northumberland, rainfall temperature and sunshine totals will be remarkably close to the long-term normal overall, but north-west Scotland will be wetter and windier than normal. Gales are likely to occur here two or three times during the second half of the month. During October the cooling Arctic with the North Atlantic liable to remain warmer than usual, combined with continued hurricane activity is likely to mean a stronger atmospheric temperature gradient between the Arctic and the North Atlantic with continued injections of moist tropical warmth from the south: This strongly suggests deeper depressions, particularly later in the month. All things considered, with the rapid cooling of the Greenland Ice-cap and NE Canada whilst the westernmost part of Canada and the North Atlantic remain relatively warm- and with the factors discussed above likely to lead to a Circumpolar Vortex around or slightly stronger than usual for the time of year, I am confident of a so-called “three-wave” pattern with an upper ridge close to the North American Rocky Mountains. This would also place an upper ridge over Western Europe with the result being high-pressure encouraged to occur a little downwind- i.e. over central Europe whilst the upper-air pattern pushes progressively deeper depressions from coastal eastern Canada north-eastwards towards Iceland and then up into the Barents Sea. Thus, during October the prevailing winds over Britain will be south-westerly, and they will increase in strength during the month. During October these winds will often bring wet and windy conditions throughout the country. However, the southern half of the country- all of England except Lancashire, Cumbria and Northumberland along with south and central Wales- will have a good deal of dry fine weather during the first ten days of October (there are indications that the weak upper-air Circumpolar Vortex will remain weaker than normal into October and the absence of a “Cold Patch” in the North Atlantic will encourage the jet-stream to blow further north on coming into NW Europe). The fine conditions will result from extensions of high-pressure from Europe across England and Wales: There will be a run of warm sunny days with maxima near 20˚C with light south or south-west winds; clear nights will again result in temperatures dropping well into single figures and local ground-frost and fog can again be expected. For Scotland, northernmost England and Northern Ireland and North Wales, October 2017 will be unsettled from the outset. Strong winds and rain will be frequent and it will get increasingly stormy. By mid-month the weather will turn unsettled even in the South with some wet and windy spells. Gales and widespread heavy rain will sweep the entire country at least a couple of times during the second half of October and localised flooding is likely to occur in western Scotland, Cumbria and Lancashire. As a couple of the deepest depressions push eastwards late in October colder strong west or north-west winds will sweep the country to bring cold squally showers (with hail locally) and daytime temperatures below 10˚C as far south as Birmingham and the first snow will fall on high ground above 600 metres in Scotland, Cumbria and Northumberland- to provide a real foretaste of winter. There is likely to be at least one night with a ridge of high-pressure following such cold strong north-westerlies when skies clear and winds fall light. Air temperatures will fall close to freezing across the entire country and ground-frost will be widespread. However, except in some rural upland areas of Scotland or sheltered frost-hollows further south, I do not expect the first air frost to occur for most of us until November. The large scale upper-air pattern is likely to bring a return to some fine dry weather lasting just a few days during the second-half of October- again this fine weather- brought about by a westwards extension of the European High will be confined to central and southern England, South Wales and Yorkshire. The fine spell is likely to be associated with cooler European air coming in from the south-east late in October and with weaker late-October sunshine daytime maxima anywhere above 16˚C are unlikely- even in the South. Clear skies at night would encourage temperatures to drop sharply to within a few degrees of freezing point over a wide area and patchy morning fog will be widespread. However, as I have already hinted above, air-frost is unlikely to occur except in some rural low-lying valleys: Most places in England will not be getting their first air-frost of the season until November. Scotland is liable to miss out on any substantial October fine spells altogether. For central and southern England, Yorkshire and South Wales I expect October to be about 1˚C warmer than the long-term normal with mean daily temperatures approaching 14˚C along the South Coast. I expect rainfall totals to be below the long-term October norm in these areas- but not by more than 40%. For Scotland, Cumbria, Lancashire and Northumberland I expect average temperatures to be a little above normal overall (by about 1˚C,- this means mean daily temperatures in lowland Scotland, Cumbria and NE England will be 9 to 10˚C) but for these areas to be significantly wetter than usual (by 30 to 60%): The exception to the wetter-than-normal regimé will be in eastern coastal parts of Aberdeenshire, Tayside and the Northumberland and County Durham Coasts where rainfall totals will be below normal for October on account of the rain-shadow effect to the prevalent south-westerly winds provided by the Pennines, Southern Uplands of Scotland and the Grampians. For November, the influence of Atlantic hurricanes will come to an end and the important factors pertaining to our expected weather will be the steeper-than-normal baroclinic gradient between the Arctic and warmer-than-usual North Atlantic that would tend towards boosting the intensity of depressions. However, the ITCZ will be drifting south over the Equator where (as things stand) sea-surface temperatures are actually cooler than normal and a weak Lá Niná is in operation. This would serve to weaken the Hadley Circulation and (with it) the strength of the upper Westerlies from the subtropics northwards. Then there is the smaller impact of factors like a quiet Sun and the now easterly QBO feeding into the Northern Hemisphere Circulation. This is (therefore) my take on how it is liable to pan out: The Circumpolar Vortex of strong westerlies encircling the Arctic will thus be strong, but not that strong- and it will manifest as the “three-wave” pattern of upper ridges and troughs during November. With an upper “ridge” anchored over the North American Rocky Mountains (through a process called Topographical Locking) this places an upper “ridge” over the UK and north-west Europe (i.e. 120˚ longitude downwind). This would produce the kind of situation where there would be high-pressure over central and eastern Europe (the upper westerlies re-curving clockwise and south-eastwards over this region would bring about high-pressure below); it would also mean deep depressions forming in the Newfoundland area and racing north-eastwards to just south of Iceland. This pattern means mild south-westerly winds over Britain with plenty of rain and gales associated with deep depressions passing further to the north that would affect the northern half of the country. However, that is not to say that rain and wind will not affect the South because the Circumpolar Vortex expands going into winter. That said, bearing in mind influences like the QBO Easterly Phase and a Hadley Circulation weakened by some cooler-than-normal waters around the Equator it is certain that the upper-air Circumpolar Vortex will be weak enough to enable high-pressure over Europe will muscle its way north-westwards to affect England and Wales at some point during November, though it will probably not extend to Scotland. To that end, I am predicting that for about a week during either mid-late November that high-pressure will bring colder drier conditions for England and Wales with the air coming from the south-east off the cooling continent of Europe. The result will be sharp night frosts with patchy freezing-fog as the air cools during the clear long November nights, minimum air-temperatures of -3˚C will occur widely with some frost hollows dipping below -6˚C. There will be sunny but cold days with weak November sunshine with maxima below 6˚C (but below freezing point where any freezing fog fails to clear). For the remainder of November, and throughout the month further north in Scotland, cloud wind and rain is likely to dominate. Heavy rain and gales will batter the west of Scotland and NW England at least a few times during the month bringing localised flooding and storm-damage. Typical daytime temperatures during these wet spells with south-west winds will range from 13˚C in the South of England to 9˚C in the Scottish lowlands; with minima just a few degrees lower at night. Snow will affect the Scottish mountains and the highest Lake District and North Pennines mountains when the wind veers towards north-westerly following some of the deepest depressions: Air-frost will be widespread on the few clear nights that will occur in “ridges” following such colder west/north-west winds. These ridges of high-pressure, occurring between the passages of depressions to the north, will result in a few bright days with weak sunshine across the entire country, though maxima are unlikely to be above 10˚C on such days, even in the South. For the month of November 2017 as a whole the mean daily temperature will be just a little above the long-term November norm across the country, ranging from about 9˚C over southern England and South Wales to 6˚C in the Scottish lowlands. Rainfall totals will be close to the long-term November norm over the Midlands, Yorkshire and the South of England, but up to 50% wetter (and windier) than normal over west and northern Scotland, Northumberland, Cumbria and Lancashire; coastal parts of NE England and eastern Scotland will have November rainfall totals close to the seasonal norm. North Wales and Northern Ireland will also be wetter than normal with upwards of six inches of rain in total falling during the month.
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